Hard-fired ceramic filters are the most common type of filter in general use in foundries today. Refractory cloth filters have been used sparingly in European foundries and have been tested in some domestic foundries. Although these cloth filters provided excellent filtration characteristics and were successful in trials, they were never promoted or developed for ferrous casting in the U.S. because of high cost and difficult availability. A refractory cloth filter of novel composition is the subject of a separate patent application by the applicant.
The conventional application of hard-fired ceramic and refractory cloth filters has been to place them at the top of the downgate of the mold, immediately underneath the pouring basin or at the base of the downgate on the mold parting line. The latest developments incorporate the filters into the runner bar; the refractory cloth filters can be placed at the ingates to the casting cavity where they can also act as knock-off cores.
In the production of most ferrous alloys and some nonferrous castings the molten metal is treated with various types of ferroalloys, inoculants, master alloys, grain refiners and deoxidants. Usually the beneficial effects of these alloy additions are short-lived; therefore, it is best to make such additions as late in the casting cycle as possible.
The general practice has been either to dribble the additive into the stream of metal entering the mold or to drop it into the downsprue before pouring the metal on top of it. Both of these methods waste alloy, do not supply an accurate weight of additive and involve high maintenance costs for the additive placement equipment. There are now no hard-fired ceramic or refractory cloth filters available for molten metal treatment with metal additives on their surfaces.